I mean there is already no AA filter in the M9, so there will be no increase in sharpness from a color filter being added.

The problem is that without the color channels you can really not work with the file, even if your final result is going to be B&W. Programs like Silver FX Pro need the color information in order to create the stunning B&W conversions it does.

I am unconvinced that this is a good idea.

In theory a B&W version of a colour sensor with the colour filter removed could give a sharper B&W image than by taking the colour image and subsequently converting it to B&W.